Nov. 15, 1883] 



NATURE 



67 



should be rigidly fixed in an upright position. The question 

 therefore arises, is this clock which deals out seconds of such 

 accuracy the only piece of mechanism that can record and 

 divide our time, or is any other time-measuring instrument avail- 

 able? Fig. 21 shows part of such an iu'-trument, known as the 

 Chronometer, in which, whilst the principles necessary to be 

 followed in the construction of. the clock have been adhered to, 

 the pendulum has been dispensed with, and the perfect stability 

 and verticality of position so important to the clock, are here 

 unnecessary. 



In this instrument the pallets of the dead-beat escapement 

 have been replaced by a detent, D. Let us consider the action. 

 The escape-wheel, s, is advancing in the direction of the hands 

 of a clock. One of its teeth meets the detent, and the wheel 

 is locked. Then what happens is this : when the balance-wheel, 

 Kj, swings, the circle, R^, centred on it shares its motion. This, 

 it will be seen, is armed with a little projection. 



We left the escape-wheel locked. Now assume that the 

 balance-wheel is swinging in the direction of the arrow. It 

 carries the small circle with it, and the piece, p,, in its motion, 

 coming into contact with the end of the spring, seen pro- 

 jecting beyond the arm of the detent, raises it and the 

 detent, so releasing the tooth of the escape-wheel. The 

 slight retardation «hich the balance receives in consequence 

 of this action is immediately compensated. The moment 

 the escape-wheel moves on again, one of its teeth meets the 

 projection, P,, and the balance-wheel receiving this fresh im- 

 pulse goes on to complete its swing. Ihen it returns and 

 swings in the opposite direction, this time without acting in any 

 way on the detent. When the balance-wheel made its first 

 swing and the point P2 met the projecting end of the spring, the 



latter could then only bend from the end of the arm with which 

 the detent is provided and against which the point Po forced it. 

 But on the return swing the spring is found capable of bending 

 from the more distant point of its attachment to the shank of the 

 locking-piece. It is therefore easily pushed aside ; there is no 

 change in the position of the detent, nor is any resistance offered 

 to the motion of the balance-wheel, which goes on to complete 

 its swing. Then another tooth is caught, the escape-wheel is 

 again locked, and again released by the lifting of the detent. So 

 the action goes on, the teeth of the escape-wheel bein^ con- 

 stantly detained and as constantly released by the action of the 

 point p„. The balance-wheel, it will be noted, receives its im- 

 pulse only at every alternate swing, whereas in the clock the 

 pendulum receives its impulse at each vibration. 



Time then can be divided down to the l/ioth of a second, or 

 as we expressed it, down to the 864,000th part of a day, not 

 only by a clock, but also by this chronometer. Having obtained 

 this i/ioth of a second by these instruments, the question 

 arises as to whether it be possible to get a still finer division. It 

 will be seen that a very much finer division than this can be 

 obtained, the l/iooth part of a second being a measurable 

 quantity ; not that such a small fraction of time as this is ever 

 necessary in astronomy, nor will it be until the present 

 asti'onomical methods have ceased to exist. If it were 

 possible to get all observations made by photography, then it 

 would be worth while recording with such minuteness, because 



photography would always behave in the same way, whereas 

 two observers never have the same idea as to the time of occur- 

 rence of any phenomena which they observe, ^'et, although so 

 great an accuracy as this is not attempted, it will be quite worth 

 while to consider the means by which this exquisite fineness of the 

 division of a second of time has been arrived at. We shall see 

 that just in the same way as an appeal to mechanical principles 

 resulted in an improvement in the construction of our clock, so 

 this fineness in the division of time has been obtained by an 

 appeal to the principles of electricity. Let it be assumed that 

 the seconds pendulum of our clock swings with perfect accuracy 

 and with absolute uniformity from second to second, in spite 

 of changes of temperature and other perturbing influences ; and 

 having assumed this, let us see how electricity can be made to 

 aid in the measurement of time. The instrument used is called a 

 chronograph. It consists of a metal cylinder revolving by clock- 

 work and covered with cloth, over which a piece of paper can 

 be stretched. Below the cylinder and parallel with it is a track 

 along which a frame carrying two electromagnetic markers or 

 prickers is made to travel uniformly by the same clock that 

 drives the cylinder. Wires connected with a battery lead from 

 one of these magnets to a clock and from the other to a key, 

 which can be depressed whenever an observation is made, and a 

 current so sent to the magnet. The effect of this is to cause it 

 instantaneously to attract its iron armature and cause the 

 pricUer with which it is connected to make a mark on the paper 

 above. 



The connection of the chronograph with the clock is as follows : 

 — The bearing shown in the middle of the diagram (Fig. 22) 

 is a continuation of the bearing on which the seconds hand of 

 the clock is supported, and there is a little wheel which does its 

 work quietly at the back of the clock in exactly the same way 

 that the seconds hand does its work quietly in front of it. What 



Fig. 22. — Electrical contact apparatus at back of clock. 



that wheel does is this. Every time that each of its iteeth — and 

 there are sixty of them — comes to the top of the wheel it touches 

 a little spring. That little spring then makes electrical contact, 

 and a current is sent flowing through parts of the apparatus already 

 described. Now the teeth in that wheel, being regularly dis- 

 posed around its circumference, always succeed one another after 

 exactly the same interval of lime, and there is no difference or 

 distinction from second to second, or from minute to minute. 

 But suppose that before the clock is started one of these teeth is 

 filed off, and so filed off that when the seconds hand points to o 

 seconds, and the minute band to a completed minute, this part of 

 the wheel shall be at the top, and there shall be no electrical contact 

 established, for the reason that the tooth of the wheel is not there 

 to act on the spring. In that way it is easy to manage matters so 

 that tbe beginning of each minute shall be distinguished from all the 

 other fifty^nine seconds which make up the minute. Let the 

 cylinder, covered with paper, revolve once in a minute. In that 

 case, the electrical current will make a hole or a mark on that paper 

 every second, and as matters are so arranged that the prickers shall 

 he travelling along at the time that the dots are made upon the 

 revolving paper they are thus made along a continuous spiral, and 

 since we^have supposed the cylinder to revolve once in a minute, 

 the beginning of each minute will be in the same line along the 

 spiral. Then, according to the length of the cylinder, a second 

 of time will be obtained written in dots, sixty of them round the 

 cylinder representing sixty seconds. Suppose now that a man 

 with a perfect eye makes an observation, recording it by sendmg 

 a current through the apparatus and making a dot on the paper. 

 He will then have an opportunity of observing on the paper the 



